The present invention relates to an air/fuel ratio detector for use in the measurement or control of the concentration of oxygen in exhaust gas from a combustion device such as an internal combustion engine or gas burner.
An oxygen sensor comprising an ion-conductive solid electrolyte (e.g. stabilized zirconia) coated with porous electrode layers (e.g. Pt porous layers) is capable of detecting combustion at a near theoretical (or stoichiometric) air/fuel ratio by sensing a change in electromotive force that is produced by the difference between the partial oxygen pressure of the exhaust gas and that of atmospheric air. This type of oxygen sensor is presently used in several applications, for example, in an automobile to run its internal combustion engine at the theoretical air/fuel ratio.
The conventional oxygen sensor produces a great change in its output if the operating air/fuel ratio (A/F ratio), which is the weight ratio of air to fuel, is at the theoretical value of 14.7, but otherwise the resulting change in output is negligible. Therefore, the output from this sensor cannot be effectively used if the engine is operating at A/F ratios other than the theoretical value.
Japanese Published Unexamined Patent Application (OPI) No. 153155/1983 discloses an oxygen concentration detector comprising a pair of oxygen ion conductive, solid electrolyte plates, each plate having an electrode layer on both sides, in a selected area close to one end. The two plates are positioned parallel to each other and spaced so as to leave a gap in an area corresponding to that selected area having the electrode layers. One electrolyte plate is used as an oxygen pump element, and the other plate is used as an electrochemical cell element that operates by the difference in oxygen concentration between the ambient atmosphere and the atmosphere in the gap between the two plates.
This type of detector features quick response, but according to the experiments conducted by the present inventors, if this device is used in a fuel-rich region having an A/F ratio lower than the theoretical 14.7, the direction of the resulting output is the same as that obtained in a fuel-lean region. Because a single output could signify more than one A/F ratio, the sensor can be used only when it is definitely known whether the combustion device to be controlled is operating in the fuel-rich or fuel-lean region.
Furthermore, it has been found that it is very difficult to use the detector at a theoretical air/fuel ratio or at a near region thereof with an accurate control of a combustion device and a quick response.